Supercritical fluid chromatography (SFC) is a type of normal phase chromatography that typically uses carbon dioxide as the mobile phase. SFC combines many of the features of liquid chromatography (LC) and gas chromatography (GC), and can often be used for separations with compounds that are not suitable for LC or GC. For example, SFC chromatography can be advantageous for separation and analysis of hydrophilic and chiral compounds, lipids, thermally-labile compounds and polymers. Other advantages include the lower cost and toxicity of the mobile phase compared to many liquid mobile phases and carrier gases.
The carbon dioxide mobile phase is generally at a pressure and temperature where the mobile phase remains in a supercritical fluid state. More specifically, the carbon dioxide mobile phase is in a supercritical fluid state at a temperature greater than 31° C. and pressure greater than 73 atm. Typically the mobile phase also includes a solvent additive. A common solvent additive is methanol. Examples of other solvent additives include acetonitrile and alcohols such as ethanol and isopropanol.
The SFC mobile phase path can exhibit problems that are generally not observed with LC and GC systems. In particular, corrosion and erosion can occur at various fluid connections in the chromatography system such as at the injector couplings and at the entrance and exit couplings of the column. For example, a mobile phase tube can internally erode near the exit end of the tube over time so that the inner diameter increases to leave an eroded region having a funnel shape at the end face of the tube. Moreover, the end faces of the tubing and surfaces of mating ports can corrode and exhibit pitting. Other locations in the system that may experience corrosion or erosion include dead volumes or regions of expanding volume. The effects of the corrosion and erosion may include the loss of pressure, loss of flow, or phase change, which affect retention times. Other negative effects may include carryover, bandspreading and changes in the shape of chromatogram peaks.